['76 911S 3.0]

I would try to set your warm base idle with the idle valve completely closed and use your base throttle blade position to get you your desired idle speed. From my experience with the Carrera idle valve, with the electrical connector disconnected, the valve is open about 25%. I have my base idle set using the blade base position (you can use the bleed screw to fine tune), and even with the idle valve completely unplugged, it will settle into a rock steady idle at 950 RPM every time.

Also make sure that there is no Idle Ignition PID active when setting the base idle (not sure if MS has this functionality), or any idle ignition modifier active. It sounds like there might be an Idle Ignition modifier going on in the background that, when it becomes active, starts chasing its tail and might be causing your oscillations and stalling.

And FWIW, my idle ignition timing is 10 degrees BTDC.